Automatic welding apparatus of this kind are known and have, inter alia, been designed and produced by applicants in cooperation with ESAB, of Gothenburg, Sweden. In this respect there is used a robot arm of our design which has six separate servo-controlled axes of rotation, or freedoms of movement, and a programmable data processing system. Seated on one end of the robot arm is an automatically controlled welding gun, which can be moved by the arm along a programmed path in three-dimensional space. The data processing system is arranged to produce at each moment, and for each rotational axis/degree of freedom, a set point value. In view of the fact that the individual servo-systems guide the end of the arm in movements which, for structural or design reasons, follow circular arcuate paths with radii of mutually different values for different arm positions, it is assumed that, for example, a programmed rectilinear path is, in actual fact, comprised of a polygonal-like path made up of small circular-arcs. Be this as it may, it is possible with this known construction to readily weld straight and curved joints, as required, with great precision.
It is often the case, however, that the workpieces to be welded have been produced with far less precision than the precision represented by the robot arm, which leads to particular problems. For example, it may be desired to weld a joint between two plates which do not meet evenly, but which deviate one from the other, due to irregularities in the plates or due, for example, to the fact that when two curved or rounded plates are to be welded together the automatic welding process requires a higher degree of precision with respect to the workpiece than is justified with respect to the precision required of the finished product.
Consequently, when welding with the aid of such automatic welding apparatus, it is desirable to be able to take into account the individual and unforeseeable shape of the workpiece, by being able to follow the actual joint to be welded, as is possible when welding manually.